In our living environment, there are a large number of malodorous molecules having different polarization characteristics and molecular weights. Hitherto, a variety of methods have been developed for reducing various malodorous molecules. Generally, the methods for reducing malodors are broadly classified into a biological method, a chemical method, a physical method, or a sensory method. Among malodorous molecules, short-chain fatty acids and amines, having high polarity, can be reduced through a chemical method; i.e., neutralization. Sulfur-containing compounds such as thiol can be reduced through a physical method; i.e., adsorption. However, there still remain many malodorous molecules, such as medium-chain and long-chain fatty acids and skatole, which cannot be reduced through known malodor reducing techniques.
In our everyday lives, among other malodors, fecal odor and foul breath are a particularly unpleasant odor. One of the main causal ingredients of such malodors is skatole. Known means for reducing a skatole odor or a fecal odor include the following: a composition containing a porous substance, an aminopolycarboxylic acid, and a metal (Patent Document 1); a silk burned product supporting a catalyst such as platinum (Patent Document 2); a deodorant containing, as an active ingredient, allyl heptanoate, ethyl vanillin, methyl dihydrojasmonate, raspberry ketone, or eugenol (Patent Document 3); and use of an aromatic component such as amylcinnamaldehyde, ethyl cinnamate, 2-cyclohexylpropanal (Pollenal II), geranyl acetone, cis-3-hexenyl heptanoate, cis-3-hexenyl hexanoate, 3-methyl-3-butenyl 2,2-dimethylpropionate (Lomilat), methylheptenone, valencene, dimethyltetrahydrobenzaldehyde (Triplal or Ligustral), cis-jasmon, acetylcedrene, benzyl acetate, geraniol, orange recovery flavor, or a plant extract of Dipterocarpaceae (Patent Documents 3 and 4).
According to the aforementioned means, a malodor is reduced by decreasing the amount of target malodorous substance through adsorption/decomposition or by means of an aromatic. However, the combination of adsorption and decomposition of a malodorous substance is not immediately effective, since the decrease of the amount thereof requires a long period of time. Use of an aromatic also has drawbacks in that the odor of the aromatic itself sometimes causes an unpleasant sensation to users, and the aromatic tends to mask odor of substances other than the target malodorous substance.
In mammals including humans, the mechanism for odorant recognition includes binding odorant molecules to olfactory receptors present on olfactory sensory neurons included in the olfactory epithelium, which is present in an upper portion of the nasal cavity, and transmitting the response of the receptors to the central nervous system. It has been reported that, 387 different olfactory receptors are present in human, and the genes encoding these olfactory receptors account for about 3% of the human genome.
Generally, a plurality of olfactory receptors respond to a plurality of odorant molecules. Specifically, one single olfactory receptor responds to a plurality of structurally similar odorant molecules at different affinities, while one single odorant molecule is detected by a plurality of olfactory receptors. It is also reported that a certain odorant molecule which can activate one olfactory receptor serves as an antagonist which inhibits activation of another olfactory receptor. Such combined response of these olfactory receptors leads to recognition of each odor.
Thus, when a first odorant molecule is co-present with a second odorant molecule, in some cases, the response of an olfactory receptor to the first odorant molecule is inhibited by the second odorant molecule. Through the inhibition, the odor of the first odorant molecule recognized by olfactory receptors may vary considerably. This mechanism is called “olfactory receptor antagonism.” Odor modulation by olfactory receptor antagonism, which differs in mechanism from a malodor reducing method by adding a perfume, an aromatic, or a like substance to the target odorant, can inhibits recognition specific to a malodor. In addition, the odor of an aromatic causing an unpleasant sensation to users can be prevented. Therefore, odor modulation based on olfactory receptor antagonism is a preferred means for reducing malodor.
In order to attain olfactory receptor antagonism, an olfactory receptor which responds to a target malodorous substance must be determined, and a substance which exhibits an antagonistic effect on an olfactory receptor of the malodorous substance must be identified. However, such identification is not easy. Hitherto, odor evaluation has been carried out through a sensory test by experts. However, the sensory test has problems. These problems include for example, odor-evaluators must be trained, and the throughput of the test is low.